UV absorbers are blended in cosmetics to protect human skin from UV radiation or to prevent degradation of components due to UV rays.
The UV wavelength range can be divided into UV-A (320 to 400 nm), UV-B (290 to 320 nm), and UV-C (up to 290 nm). Among them, UV-C ray generally does not reach the earth's surface.
UV-A ray (320 to 400 nm) has not been thought to accelerate skin aging by causing sunburn such as UV-B ray (290 to 320 nm) though it blackens the skin. Recently, however, it has been revealed that UV-A ray reaches deep into the skin and causes not only skin aging but it also promotes the formation of skin cancer while UV-B ray only reaches relatively surface layers of the skin.
When classifying according to molecular structure, conventional UV absorbers for cosmetics can be classified into (1) benzoic acid derivative, (2) cinnamate derivative, (3) benzophenone derivative, (4) dibenzoylmethane derivative, (5) salicylic acid derivative, (6) camphor derivative, (7) phenylbenzimidazole derivative, (8) phenylbenzotriazole derivative, (9) triazine derivative, (10) phenyl acrylate derivative, and so on.
Among the above-mentioned absorbers, (4) dibenzoylmethane derivative is one of the most widely used UV absorbers in cosmetic field in recent years because it is excellent in safety and UV absorbing ability (especially UV-A absorbing ability). One of the representative examples of dibenzoylmethane derivative is 4-tert-butyl-4′-methoxydibenzoylmethane which is the most used UV-A absorber because it has absorption maximum around 360 nm as well as a large absorbance and is excellent in absorbing UV-A ray.
However, the dibenzoylmethane derivative has a low photostability. Therefore, a preparation containing dibenzoylmethane derivative has had a problem in that its UV protective effect is deteriorated over time by exposure to UV ray or sunlight. Thus, in order to exert a constant skin protecting effect for many hours under light, it was necessary to apply the preparation containing dibenzoylmethane derivative to the skin or hair steady or repeatedly within a short period of time.
To try to solve the above-mentioned problem, Patent Literatures 1 and 2 disclose that certain kinds of alkyl β,β-diphenylacrylate and α-cyano-β,β-diphenylacrylate, which are used as UV absorbers, improve photostability of the dibenzoylmethane derivative. For details, Patent Literature 1 discloses a light-blocking cosmetic composition which stabilizes the UV-A blocking agent by blending the UV-A blocking agent (such as dibenzoylmethane derivative) with an alkyl β,β-diphenylacrylate or an α-cyano-β,β-diphenylacrylate in a specific proportion. Patent Literature 2 discloses a light-blocking cosmetic composition with photostability which is prepared by blending a benzoylmethane-type UV-A blocking agent with a α-cyano-β,β-diphenylacrylate stabilizer in a specific proportion to stabilize the benzoylmethane-type UV-A blocking agent and adding arbitrarily at least one kind of general UV-B filter. In Patent Literature 2, a particulate pigment (e.g. a micro pigment of a general metal oxide) is specified as the above-mentioned UV-B filter.
However, there has been a problem in that the effect of the alkyl β,β-diphenylacrylate and the α-cyano-β,β-diphenylacrylate to stabilize a dibenzoylmethane derivative is inhibited by blending a cinnamate derivative (such as octyl methoxycinnamate), which is one of the widely used UV absorbers in cosmetics, especially as a UV-B absorber, into a system using a dibenzoylmethane derivative as disclosed in the Patent Literatures in combination with alkyl β,β-diphenylacrylate or α-cyano-β,β-diphenylacrylate.
If it is possible to blend a UV-B absorber having an excellent UV absorption (especially for UV-B ray) into a system containing a dibenzoylmethane derivative (i.e. UV-A agent) and an alkyl β,β-diphenylacrylate, α-cyano-β,β-diphenylacrylate and similar stabilizers (i.e. stabilizers capable of enhancing photostability of the dibenzoylmethane derivative) without deteriorating the effect of the alkyl β,β-diphenylacrylate and similar stabilizers to photostabilize the dibenzoylmethane derivative, a UV absorption for a wide range of UV wavelength from the UV-A ray to the UV-B ray can be achieved stably. The development of such products has been desired.
As far as known to the inventors of the present invention, there has been no literature specifically disclosing the way to solve the problem in that the use of cinnamate derivatives inhibits the effect of the alkyl β,β-diphenylacrylate and the α-cyano-β,β-diphenylacrylate on the stabilization of dibenzoylmethane derivatives.    [Patent literature 1] Japanese Patent No. 2975682    [Patent literature 2] Japanese Unexamined Patent Publication H09-175974